1. Field of the Invention
The present invention relates generally to a brake band for an automatic transmission and, more particularly, to a brake band for an automatic transmission that employs a two-stage solenoid for providing an initial fast apply pressure to the band and then a PID controlled apply pressure to the band based on measurements of the brake drum speed and band loading.
2. Discussion of the Related Art
Automatic transmissions used in modern passenger vehicles typically employ both band shifting mechanisms and friction disk or clutch pack shifting mechanisms. The relatively simple hydraulic control systems that were used at the time automatic transmissions were originally designed made the choice between band shifting mechanisms and clutch packs dependent upon packaging, to a great degree. More modern designs, however, have employed more sophisticated control strategies. Band shifting mechanisms have typically been avoided in these newer designs because of concerns with band response time and variability as compared to clutch pack shifting mechanisms. Band shifting mechanisms do, however, have advantages of durability and spin loss over clutch pack shifting mechanisms.
The environment that a band shifting mechanism operates in is not as controlled as a clutch pack shifting mechanism environment. Lubrication variation, actuation fluid volume, and system stretch are variables that cannot easily be handled with mapped responses in band systems. It has, therefore, been a goal in the art to allow transmission designers the same packaging flexibility previously seen in legacy transmissions without compromise of modern controls or customer satisfaction.
U.S. Pat. No. 6,110,068 discloses a synchronized shifting technique that employs two separate friction elements for use in a brake band or friction disk transmission. The friction elements are monitored during the shift and adjusted to provide an improved gear ratio shift. While such a technique may provide improved transmission shifts, it lacks certain characteristics that allow precise control of the band during the shift. A more precise control of the brake band would provide an even smoother and more desirable shift of the transmission.